(1) Field of the Invention
This invention relates to a method for protecting high temperature areas of equipment or facilities auxiliary to a fluidized incinerator from corrosion. Such high temperature areas include, for example, metallic areas exposed to combustion gas of high temperature such as cyclone dust collector, air preheater, waste heat boiler, their piping and the like which are provided in connection with the main body of the incinerator.
(2) Description of the Prior Art
A fluidized incinerator has such merits that it can handle a larger volume of waste materials per unit area and it promotes their perfect combustion. Fluidized incinerators have thus found wide-spread commercial utility as incinerators for municipal and industrial waste. These municipal and industrial wastes, however, contain chlorine-containing compounds and, upon burning, produce hydrogen chloride (HCl) gas and the like, thereby subjecting incinerators and their auxiliary equipment or facilities to corrosion and shortening their service life to impose big limitation on the recovery of theremal energy. It has thus been attempted to solve the above problems by using a heat and corrosion resistant steel material as a structural material or by supplying an alkaline compound into fluidized incinerators to neutralize and remove acidic gas produced in the course of combustion.
However, even if the acidic gas is neutralized and its concentration in the combustion gas is lowered to any considerable extent, it has been recognized that corrosion still takes place, especially at ash-covered areas because metals are heated to 450.degree. C. or higher at their surfaces in the equipment, facilities and piping auxiliary to a fluidized incinerator. If one wants to avoid corrosion at high temperatures, it is necessary to keep the temperature at the surface of a steel material below 450.degree. C. For this purpose, it has been a practice to lower the temperature of the combustion gas by spraying water or blowing cold air into the combustion gas or by increasing the flow velocity of a heat transfer medium in each heat exchanger. These measures, however, impose tremendous limitations when recovering energy from the combustion gas. For example, in the case of recovering the energy of combustion gas in the form of electric power by means of a steam turbine, lowered surface temperatures lead to a lowered pressure and a reduced volume of steam generated, thereby lowering the efficiency of heat recovery.
A variety of investigations and research have heretofore been done with respect to high temperature corrosion of incinerators. It has been known that the presence of HCl in combustion gas accelerates the corrosion and induces violent corrosion at temperatures above a certain level. The mechanism of such corrosion has already been elucidated. Namely, Fe is reacted with HCl to form FeCl.sub.3 and Fe.sub.2 Cl.sub.6, followed by decomposition of these iron chlorides to iron oxides with spontaneous regeneration of HCl. HCl is also formed due to the decomposition of chlorides such as NaCl, KCl, CaCl.sub.2 and the like present in incineration ashes. The thus-regenerated HCl seems to play the principal role in metal corrosion.
As counter-measures against high temperature corrosion due to hydrochloric acid, it may be effective:
(1) to periodically replace corroded parts of auxiliary equipment, considering them as expendable parts;
(2) to apply a special protective material on the surface of each part which is susceptible to corrosion; and
(3) to use a high-quality corrosion-resistant material.
However, it has been reported that counter-measure (1) requires frequent interruption of the operation of the incineration plant or the installation of standby equipment for each item of equipment which is liable to undergo corrosion; and counter-measure (2) does not show any noticeable effects even if Al.sub.2 O.sub.3 or its analogous refractory material is applied as a protective coating. As to counter-measure (3), the anti-corrosion effect of high chromium steel has been reported. However, such materials are costly and involve certain problems in their mechanical properties, thereby making themselves unsuitable for practical use. Under such circumstances, it is common practice to avoid the development of high temperature corrosion by, for example, lowering the temperature of each metal surface which is brought into contact with combustion gas of elevated temperature, as mentioned above.